Previously known blood separating agents of this type include various blood separating agents obtained by adding a specific gravity adjusting agent, a thixotropy enhancer, or a viscosity modifier to a resin. Patent Literature 1 below discloses, as an example of the resin, a mixture of a hydrogenated cyclopentadiene-based petroleum resin solid at ordinary temperature and a plasticizer. Silicone polymers, α-olefin copolymers, acrylic copolymers etc. are also conventionally used.
Examples of the specific gravity adjusting agent used to be added to the resin include inorganic powders, such as silica. On the other hand, examples of the thixotropy enhancer or viscosity modifier for use include various organic compounds. These organic compounds having been heretofore used include glycerin, propylene glycol, ethylene glycol, ethylenediamine, dibenzylidene sorbitol, fatty acid amide, fluorocarbon-based surfactants, polyester-modified alkyl polysiloxane-based surfactants, polyether-modified alkyl polysiloxane-based surfactants, and ethylene glycol-propylene glycol block copolymer-based surfactants.
Furthermore, in using such a hydrogenated cyclopentadiene-based petroleum resin solid at ordinary temperature as exemplified in Patent Literature 1, it is necessary to dissolve the hydrogenated cyclopentadiene-based petroleum resin in a plasticizer to convert it into liquid form at ordinary temperature. Examples of the plasticizer include phthalic acid esters, trimellitic acid esters, sebacic acid esters, and maleic acid esters. However, such a resin component composed of a solid resin and a plasticizer considerably varies in viscosity with temperature owing to the molecular structure of the hydrogenated cyclopentadiene-based petroleum resin, so that the viscosity of the resultant separating agent will be high at low temperatures. Therefore, there arises a problem in that when a cooling centrifuge is used, the fluidity of the blood separating agent will be reduced.
Alternatively, if a silicone polymer is used as the resin, it will react, during long-term storage, with the surfaces of silica particles added as a specific gravity adjusting agent thereto, and consequently the thixotropy of the resultant blood separating agent may be reduced.
On the other hand, in a blood separating agent in which an α-olefin copolymer is used, a large amount of inorganic powder has to be added as a specific gravity adjusting agent because of a small specific gravity of the α-olefin copolymer itself. Therefore, during centrifugation, the inorganic powder may be separated from the resin by a centrifugal force, so that no partition wall may be formed between clot and serum or between blood cell components and plasma. Furthermore, if the amount of inorganic powder added is too large, there arises a problem in that an increase in the hydrogen bonding strength during long-term storage occurs, which in turn increases the viscosity of the resultant separating agent and reduces the fluidity thereof during centrifugation.
If an acrylic copolymer is used, it will generate radicals during sterilization by exposure to γ radiation because of low resistance of the acrylic copolymer itself against exposure to radiation, so that crosslinking caused in the process of progress of the polymerization reaction and the process of main chain breakage and rebinding may increase the viscosity of the acrylic copolymer. Therefore, a satisfactory partition wall may not be able to be formed between clot and serum or between blood cell components and plasma. An example of a method for reducing the increase in viscosity is disclosed in Patent Literature 2 below. In Patent Literature 2, a highly water-soluble polymerization inhibitor, such as hydroquinone, is used. However, if a highly water-soluble polymerization inhibitor is used, the polymerization inhibitor will be eluted from the blood separating agent into serum or plasma. Therefore, there arises a problem in that when the serum or plasma is used as a specimen, its test values will be influenced. In Patent Literature 3, α-methylstyrene dimer serving as a chain transfer agent is added. However, the chain transfer agent presents a problem in that because it has the effect of inhibiting the polymerization in polymer main chains but does not allow radicals to disappear, it is not suited for fundamental reduction of the increase in viscosity.
Furthermore, most of the organic compounds used as thixotropy enhancers or viscosity modifiers are hydrophilic. Therefore, there arises a problem in that the organic compounds are eluted into the blood to damage blood cell membranes. As a result, enzymes and the like contained in large amounts in blood cells, such as LDH and AST, may leak out into plasma or serum. This may have an adverse effect on test values for the plasma or serum. In addition, there is a problem in that absorption of water in blood into the blood separating agent may be promoted to make the blood separating agent cloudy.